This invention relates, in general, to the reduction of the spurious signals created from the digital representation of analog signals. More specifically the invention relates to a technique which suppresses the spurious signals created from the quantization of an analog signal. Although the technique can be used to enhance the spur-free performance of any digital sampling/storage/reproduction system it is particularly useful in digital radio frequency memories (DRFM) which are suitable for the coherent storage and reproduction of radio frequency (RF) signals for use in military active electronic countermeasure systems, such as active radar jammers.
Active radar jammers are used in the field of electronic countermeasures to confuse or counter a system originating radar signals. In some situations, it is desirable to return signals to the radar system which are exact copies of the arriving radar signal. In other situations, it is desirable to return signals to the radar system which have characteristics other than that o the received radar signal in order to further confuse the originating radar system. In any event, it is usually necessary for the countermeasures system to store the received radar signal and reproduce it at a later time.
Previously, delay lines of various types have been used to effectively store the received radar signal for a short period and make the stored radar signal available at a later time. Typical delay lines, however, have the disadvantage that the delay cannot be electronically changed easily, and it is difficult to obtain a long delay without serious signal degradation. An improvement over delay line technology has been achieved by the use of digital radio frequency memories (DRFMs) which convert relatively high radio frequency (RF) signals down to a lower intermediate frequency (IF) by mixing the RF with a local oscillator (LO) signal; the resulting IF signal is then stored in a digital memory device. The digital memory can be controlled in a manner similar to the control of the digital memory of a computer. Stored values representing the radar signal can be recalled and reproduced at any desired time delay. Further, manipulation of the digital values to produce changes in the replicated signal are also conveniently done by digital processes.
U.S. Pat. No. 4,713,662 entitled "Modulated Digital Radio Frequency Memory", which was issued Dec. 15, 1987 in the name of Richard J. Wiegand, one of the inventor's herein, and which is assigned to Westinghouse Electric Corporation, the assignee herein, describes single and dual channel DRFMs. In either system it is necessary to first convert the RF signal down to a lower IF signal which is manageable. The IF signal is digitized by means of an analog-to-digital (A/D) converter at a given sampling rate which is determined by the capacity of the A/D equipment. The digitized signal is then stored in a digital memory. At a later time the digital signal may be called from memory and converted to an analog IF signal by a digital-to-analog (D/A) converter. The IF signal is mixed with the LO signal to convert the IF to the higher RF signal which is a replication of the incoming radar signal.
Although conversion of the incoming radar signal to a lower frequency IF signal allows more manageable manipulation of the information, a consequence of mixing signals is the production of two resulting signals which represent the sum and the difference of the original signal and the LO. In many situations the sum and the difference signals are easily distinguished. For example, if a 3100 MHz RF signal is mixed with a 3000 MHz LO signal, the resulting sum and difference signals are 6100 MHz and 100 MHz respectively. These signals are easily distinguished. In the DRFM the 6100 MHz signal is filtered out and the relatively low 100 MHz IF signal is digitized and stored in the memory. However, when the 100 MHz IF signal is recalled from memory and converted up to the RF level by mixing with the 3000 MHz LO signal, the original 3100 MHz RF signal representing the sum of the LO and IF is produced as well as a 2900 MHz image signal which represents the difference of the two. The image signal is not easily distinguished from the original signal. Thus, it is necessary to suppress the image signal.
The usual method of suppressing the image signal in DRFMs is to employ a two quadrature channel or so called I&Q memory system described in the above noted Wiegand patent. By using the two channel I&Q system the image signal can be suppressed or nearly cancelled at the output. However, I&Q systems have additional hardware components which can be expensive and heavy. Also, the described two channel system contains a non-performance region, or hole, which occurs when the frequency of the RF signal is close to the frequency of the LO of the memory system. This hole is caused by the low frequency gain roll off inherent in IF amplifiers.
According to the well known Nyquist Sampling Theory, the maximum usable instantaneous bandwidth (IBW) of a memory system is equal to one-half the sampling rate of the A/D converters used in the DRFM. Having a large instantaneous bandwidth (IBW) is advantageous from the standpoint that it allows radar signals over a wider range to be detected, stored and jammed by the countermeasures equipment. One way to maximize instantaneous bandwidth is to use one bit sampling.
One bit sampling also provides the advantages of increased amplitude dynamic range and reduced storage requirements. However, one bit sampling results in a large number of spurious frequencies or unwanted spectral lines (spurs) in the reproduced IF signal. The spurs are ultimately reproduced in the RF output signal when the IF is mixed to RF. Spurs degrade system performance and must be suppressed by some additional means.
Prior to Wiegand's invention, the most effective way to obtain reasonably large bandwidths and to suppress images was to use multiple bit sampling and the two channel I&Q system. Wiegand's patent shows that a single channel DRFM which has one bit sampling and phase or frequency modulation imposed on the LO signal can effectively reduce or decorrelate the image and spurs. However, certain higher order spurs may not be decorrelated if digitally generated phase modulation is used to modulate the LO.
In Wiegand's patent, four bit phase modulation is employed. The LO signal is phase modulated with a decorrelation waveform pattern which is later duplicated during replication of the RF output. However, pure digitally generated phase modulation will no decorrelate every spur line. Although modulation is imposed on every single potential spectral line, for some spectral lines the phase step results in a phase change in one or more multiples of 360.degree.. Hence, the modulation is not effective and the spur line appears coherently in the output.
Multibit sampling is advantageous because it provides a more accurate picture of the incoming waveform. However, not all spurs are eliminated by a finite number of bits per sample because when the signal is reproduced it is not an exact replica of the incoming signal but an approximation with many harmonics which are spurious signals.
A disadvantage of multibit sampling is that it decreases the dynamic range of the system. This occurs because the input amplitude must be controlled to be within the maximum level of the sampler. In a one bit system, only positive and negative transitions are detected. In the multibit system, amplitude information is detected and quantized. In a multibit system, the amplitude is quantized because only a limited number of amplitude resolution bits or amplitude levels are available. Further, multibit systems require additional memory and a correspondingly more complex system of implementation. Further, multibit sampling reduces the instantaneous bandwidth of the system because of the slower sampling clocks used.